1. Field of the Invention
The present invention relates to a surface mount type unit to be surface-mounted on a substrate and a transducer assembly using the same, and more particularly to a unit structure useful for reliably mounting a large self-weighted device, such as a large semiconductor package, and a transducer assembly structure using the same.
2. Related Art
Conventionally, various methods involving wire bonding, tape carrier bonding, or flip-chip bonding, or others have been known as techniques for mounting in high density a semiconductor package, such as an IC, LSI, etc., on a printed board.
In particular, since surface mounting by flip-chip bonding is a method in which the semiconductor package is directly bonded on a printed board through fine solder bumps, attention has been recently drawn to this method as a technology for enabling high-density mounting. There is illustrated as a reference in FIGS. 16A and 16B one example of such a surface mount type semiconductor package as well as a state of mounting the same on a printed board.
As shown in FIG. 16A, such a surface mount type semiconductor package 9 usually has in a backside thereof a number of signal electrodes 91 which are electrically connected to input/output terminals or electricity supply terminals of an inside semiconductor device. And, these electrodes 91 and component mounting lands (substrate electrodes) 31 provided on the printed board 3 are to be placed into bonding through solder bumps 4 in a manner shown in FIG. 16B. Incidentally, to surface-mount the semiconductor package 9 through the solder bumps 4, a reflow method using a solder paste is generally adopted.
Although this reflow method is categorized as hot air reflow, infrared-ray reflow, vapor-phase soldering, and so on, basically in either case the semiconductor package 9 is mounted in a procedure of:
(1) a solder paste is print-applied to lands 31 on the printed board 3. Incidentally, there may be a case that the same paste is print-applied also to the signal electrodes 91 side of the semiconductor package 9;
(2) the semiconductor package 9 is mounted by positioning onto the print-applied solder paste on the printed board 3;
(3) in this state, the solder is melted by heating, to bond in batch each of the lands 31 and each of the signal electrodes 91.
According to the surface mounting of a semiconductor package as above, higher density of mounting becomes possible by an amount of unnecessary mount areas for wires or tape carriers or the like, compared to the above-stated wire bonding or tape carrier bonding.
Furthermore, in the wire bonding or tape carrier bonding, the signal electrodes are disposed on an outer periphery of the package and accordingly the number of connectable electrodes are naturally limited. On the other hand, in the case of the surface mount type semiconductor package, the signal electrodes can be disposed on the entire surface of the bottom of the package. Therefore, the restriction in the number of connectable electrodes is greatly relieved. Consequently, according to the surface mount type semiconductor package, the mount density on a printed board can be greatly improved and the number of connectable electrodes is drastically increased, realizing an extremely efficient mount structure.
However, being effective mount structure as above, the integration for semiconductor devices further proceeds as the recent electronics technology rapidly advances and their packages become larger, giving rise to the following new problem resulting from the self-weight of the package.
That is, if the self-weight of the package increases, the solder thickness at the joined portions between the signal electrodes and the lands (substrate electrodes) is thinned and thermal stresses of the joined portions have been increased. The increase of thermal stresses eventually causes the joined portions to be disconnected, etc., becoming a factor of impairing the reliability for the joined portion.
As a result, for an apparatus such as an electronic control device installed on an automotive vehicle, placed in severity of temperature change or vibration and requiring high reliability, the adoption of such a semiconductor package and a mount structure thereof have been rendered difficult.
Incidentally, as seen in electronic component mount structures described in Japanese Patent Application Laid-Open No. Hei. 5-218134 or Japanese Patent Application Laid-Open No. Hei. 7-74450, if balls or spacers or the like are separately disposed between an IC chip or a leadless component and a substrate, the thinning of the solder thickness at the joined portions becomes certainly avoided.
However, in this case, auxiliary members such as these balls or spacers or the like as well as the process for providing the auxiliary members are separately required, imposing large bearing in respect of device cost as well as manufacturing cost and naturally complicating the mount structure thereof.